Double superheterodyne receivers are often used as wide or broad band receivers. An input carrier frequency is changed twice in such a receiver. There are basically two ways for effecting such double conversion in a double superheterodyne receiver. According to the first method the incoming carrier frequency, which may include an audio or video signal, is lowered to a first intermediate (IF) frequency value by being mixed with a first local oscillator frequency in a first mixer. The first IF is reduced to a second IF value by being mixed with a second local oscillator frequency in a second mixer. On the other hand, according to the second method the incoming carrier frequency is increased by a first mixer so as to be out of the receiving range, and the increased frequency is then lowered by a second mixer.
The present invention relates to a double superheterodyne receiver in which the latter method is adopted. One example of such a double conversion superheterodyne receiver is disclosed in a U.S. Pat. No. 3,939,429. In a double superheterodyne receiver as disclosed in the above mentioned patent, the incoming carrier frequency is first amplified by a wide-band high frequency amplifier and then converted twice in first and second mixers prior to being applied to an IF amplifying stage. The mixers are constructed of double balanced systems. Transmission lines between the high frequency amplifier and the IF amplifying stage are constructed of balanced circuits. Furthermore, a band-pass filter connected between the first and second mixers comprises a magnetic resonance circuit, such as a YIG device. According to the technique of the above mentioned patent, the circuitry is complex in construction since significant a number of balanced circuits is required. Furthermore, the band-pass filter has to be equipped with an external circuit for compensating for temperature variation, resulting in further complex construction. Such a complex construction of circuitry necessarily requires a number of elements, whereby the receiver is apt to be bulky and relatively high in cost.